Conventionally, CDMA has been used to overcome non-white interference—the spread spectrum nature of CDMA increases the signal-to-noise plus interference ratio (SNIR) at the point of bit decision in the receiver via the processing gain resulting from spectrum spreading and de-spreading. However, the conventional method both accepts some interference from other users and causes some interference to others, albeit relatively small depending on the processing gain. In some applications, the above reduction of mutual interference by conventional spread spectrum processing may be insufficient to meet the SNIR requirement.
Additionally, there are some conventional spread spectrum methods, such as Bluetooth™, where the spectrum of non-white interference is avoided in the design of the transmit signal. For example, the Bluetooth™ system can sense the presence of non-white interference and avoid using spectra occupied by the interference in selecting its own frequency hopping pattern. However, Bluetooth™ uses non-coherent frequency hopping, that is, in the process of making bit decisions, signal energy is not coherently integrated over multiple frequency hops. Conventional non-coherent spread spectrum systems offer inferior performance as the available signal energy is used sub-optimally.